Recently, I went through a question where there was quite alot of code and I didn't have time to read it all carefuly (I still went through the code!). I still posted an answer that reviews the code that I read, is it okay to do this? Should I tell the user that I didn't have time to read the whole code block (which I did) meaning there might be other potential problems in the OP's code or should I avoid answering because my answer isn't as complete as it could be?
Any and all aspects of the code are up for review. As such, it's okay to only review part of the code. This is particularly true in cases like you've mentioned. Those where the code in question is especially large. Review what you can, someone else may or may not comment on the rest. Either way, OP's code is better for receiving some feedback.
The other half of your question is, do you need to write a disclaimer that you only looked at part of the code? In general, I think that you don't need to, since there is no expectation that you will have spotted everything that may be wrong with the code. Just make sure that you avoid stating stating anything that explicitly praises something you didn't really look at carefully.